What is a tamarind
Used in Indian and Asain cooking. It's sold mostly at Asain markets. Either in a seedless paste, or a paste with seed pods. Both you need to soften a bit and with the seeded version you need to remove the seeds.
It's a tart, salty, earthy, a fruity flavor with undertone of Umami like you'd find in soy sauce.
It's the fruit of an African native tree, although now a lot of them are grown in Asia because, as Sam says, they're used extensively in its cuisine. It was introduced into Mexico and South America a long time ago, too, and is heavily used there.
A fruit used usually in paste form and in a variety of ways as identified by Sam and Diana. The flavor profile is sweet, sour, acidic. It is high in sugar and B-vitamins. Also, I see you are a Whole Foods Market customer. Check with your local WFM. It is very likely you can find it there in the Asian set of grocery department or fresh pods may be available in produce depending on your needs.